The present invention relates to a method of analysing seismic signals and in particular to a method of analysing seismic signals adapted for use in connection with marine seismic data acquisition activities that provides for improved determination of local wave heights and acoustic sensor depths and allows xe2x80x9cnoisexe2x80x9d in seismic data associated with changes in local wave heights and seismic sensor depths to be reduced during subsequent data processing.
Seismic data is collected to remotely sense subsurface geologic conditions, particularly in connection with the exploration for and production of hydrocarbons, such as oil and natural gas. To gather seismic data in a marine environment, acoustic sources, such as airguns, are used to produce an acoustic signal that is transmitted through the seawater and into the subsurface geologic formations. Changes in acoustic impedance at the sea bottom and between different geologic layers cause a portion of the acoustic energy to be reflected and returned toward the sea surface. These reflected signals are received by acoustic sensors and are processed to create images of the subsurface geology.
In a marine environment, these acoustic sensors (also called seismic sensors, often pressure sensors known as hydrophones) are typically contained within long tube-shaped streamers and are towed behind a seismic survey vessel. The streamers are often filled with kerosene or other buoyant materials that allow the sections of the streamers to achieve approximately neutral buoyancy. The streamers often have one or more internal stress members (such as steel cables) that provide substantial tensile strength and inhibit stretching of the streamer sections, while simultaneously allowing the streamer to be relatively flexible and able to be wound around a drum of a reasonable diameter on the seismic survey vessel. The depth (or xe2x80x9celevationxe2x80x9d) a streamer is towed at is typically regulated by a deflector located at the end of the streamer nearest the seismic survey vessel (see, for instance, our U.S. Pat. No. 5,357,892) and by control devices called birds that are typically placed at regular intervals along the streamer""s length (see, for instance, our published PCT International Application No. WO 98/28636).
The depths of the hydrophones in the streamer are typically monitored on the seismic survey vessel by depth sensors attached to the birds. Because the birds are widely spaced along the streamer (such as every 300 meters), compared to the significantly closer hydrophone spacing (such as a group of hydrophones every 12.5 meters), the depth of a particular acoustic sensor or a group of acoustic sensors must typically be approximated by interpolating from the depth values of the birds on either side of the sensor or sensor group.
This type of relatively crude depth determination system makes it difficult for a seismic survey vessel crew to determine when certain types of problems are occurring within the streamers. For instance, streamer sections are typically xe2x80x9cbalancedxe2x80x9d until they are approximately neutrally buoyant. Due to temperature changes on the seismic survey vessel and in the sea water, balancing problems (excessive positive or negative buoyancy) sometimes occur. If the depth of each of the hydrophone in each section could be monitored, however, it may be possible to determine which sections are experiencing balancing problems and to correct these problems before they impact the quality of the seismic data acquired or cause towing problems.
Depth sensors on the birds typically sense the local ambient water pressure and convert this pressure reading into a depth value. The water pressure measured at the bird, however, incorporates two types of transient conditions that are constantly changing as the streamer is towed. The first transient condition is the local wave height, the local sea level immediately above the sensor minus the mean sea level. Changes in the local wave height are also referred to as waves. The second transient condition is the actual streamer elevation (or depth) measured with respect to mean sea level. Changes in the actual streamer elevation are typically due to forces such as positive or negative buoyancy in the streamer sections, wave-induced forces, currents, the deflector, the birds, etc. The water pressure at the bird is influenced by both of these transient conditions. To eliminate wave effects, the measured water pressure values are typically averaged or filtered over an extended period of time (such as between 10 and 100 seconds). While this averaging or filtering produces more accurate xe2x80x9caveragexe2x80x9d depth values for the birds, it eliminates any possibility of using the measured depth values to compensate for transient conditions having a cycle period less than half the averaging period or filter length, such as waves.
Two types of xe2x80x9cnoisexe2x80x9d are introduced into the data by the fluctuations in the streamer depth and the local wave height. A first type of noise is caused by ghost effects. Acoustic reflections from the sea surface above an acoustic sensor or an acoustic source will cause cancellation of the received acoustic signals at frequencies that are related to the depth of the sensor or source (i.e. the xe2x80x9cghostxe2x80x9d effect). Ghosts are notches in the frequency spectrum that occur at frequencies F=n/Tg, where n is an integer (0,1,2, . . . ) and the ghost period Tg is equal to twice the receiver (or source) depth H (distance to the sea surface) divided by the seawater acoustic transmission velocity. The depth H (and therefore the ghost notch frequency F) needs to be corrected for the angle of incidence (as will be discussed in more detail below). There are two ghosts, one introduced on the source side and one introduced on the receiver side. Variations in the ghost notch frequency occur when the depth of the receiver or source varies. These variations can be due to a change in the absolute elevation of the streamer or the source or due to changes in the wave height above the streamer or the source.
To compensate for this ghost effect, seismic sensors are typically towed at a depth where the first non-zero ghost notch frequency is outside the seismic spectrum (between approximately 5 Hz and approximately 80 Hz) where the vast majority of information regarding the geologic subsurface of interest is obtained during a seismic survey. A deconvolution procedure can be used to compensate for the frequency-dependent attenuation of the received seismic signals caused by the ghost effect (i.e. xe2x80x9cde-ghostingxe2x80x9d the data). In conventional seismic data processing procedures, however, this deconvolution procedure will assume that the seismic sensors are placed a constant distance beneath the sea surface. Any deviation in the position of the sensor from this assumed position will cause the de-ghosting procedure to operate to some degree improperly; certain frequencies will be over amplified and certain frequencies will remain under amplified. In that the depth values are averaged or otherwise filtered over an extended period of time to remove wave effects on the depth values, the depth values provided by conventional seismic data acquisition equipment cannot be used to provide customised or individualised de-ghosting of the seismic data to account for the actual (and changing) depth values of the sensors when they were receiving the seismic data of interest.
A second type of noise is due to changes in the absolute elevation of the streamer which causes unintended shifts in the arrival times of the acoustic signals received from the underlying seismic reflectors. As the vast majority of seismic data analysis involves combining together numerous seismic traces imaging the same subsurface position, these time shifts will cause a blurring of the seismic image of the reflectors.
While these two types of deviations do not introduce xe2x80x9cnoisexe2x80x9d in its conventional sense (i.e. unwanted signals that interfere with or mask the desired signals), it will be readily understood that they inhibit proper seismic imaging of the subsurface and therefore constitute noise in its more general sense. For some types of seismic imaging, such as analysing time-lapsed images of producing hydrocarbon reservoirs, these effects may be sufficient to mask any change in the seismic response that could be expected to result from the withdrawal of reservoir fluids. A study conducted on behalf of the Applicant has concluded that if conventional seismic data processing schemes are utilised, rough sea effects from only a 2 meter significant wave height (SWH) sea can mask any changes in seismic response that could be expected to be associated with hydrocarbon production, at least for certain reservoir types.
In conventional marine seismic surveying, the only attempts made to compensate for changes in local sea height involve compensating for changes in mean sea level due to tidal effects. No attempt is made to correct the seismic data for wave effects or short cycle-time variations in the streamer depth values. While it is well known that the quality of seismic data will be seriously degraded if the seismic data is acquired during rough sea periods, no attempt is normally made to compensate for these type of transient conditions. When a seismic survey vessel crew or their client""s onboard observer decides that the sea conditions are too rough or fail to meet the agreed upon contractual specifications, acquisition of seismic data by the seismic survey vessel is simply stopped. The client is simply forced to live with the fact that seismic data acquired during rougher sea conditions is noisier (i.e. of lower quality) than seismic data acquired during calmer sea conditions.
Seismic data acquisition contractors have a significant incentive to acquire seismic data under xe2x80x9cquestionablexe2x80x9d weather conditions because they are not typically compensated for downtime resulting from bad weather and the amount of time spent down for bad weather can easily range between 10% and 50% of the entire mobilisation period. Some seismic data acquisition contractors are particularly aggressive about continuing seismic data acquisition activities in bad weather. This is particularly true when the seismic survey vessel is acquiring multi-client data. Multi-client data is typically acquired xe2x80x9con-specxe2x80x9d with the seismic contractor paying for the cost of the acquisition activities and then attempting to recoup these costs and make a profit by licensing access to the acquired seismic data. Some contractors apparently believe that the effects of bad weather can be removed (or at least masked) during subsequent data processing or that the clients may not realise how noisy the data actually is. This situation has been further complicated in the past because clients have lacked a method for independently determining what the sea state was when the seismic data was acquired.
It is therefore an object of the present invention to provide for an improved method of analysing seismic signals.
An advantage of the described embodiment of the present method is that it allows local wave heights and acoustic sensor elevations to be determined in connection with marine seismic data acquisition activities.
A further advantage of the described embodiment of the present invention is that it provides an objective method for determining the local wave heights directly from seismic data.
Another advantage of the described embodiment of the present invention is that the elevations of individual acoustic sensors or arrays of acoustic sensors may be determined in the absence of conventional water-pressure-based depth sensors.
An additional advantage of the described embodiment of the present invention is that xe2x80x9cnoisexe2x80x9d introduced into the seismic data by changes in local wave heights and/or changes in seismic sensor elevations may be attenuated during subsequent data processing.
The present invention involves a method of analysing seismic signals acquired by a plurality of submerged seismic sensors in response to operation of an acoustic source during a marine seismic survey, the method comprising, for each of at least some of the signals, the steps of: selecting a time window within the signal which frames a relatively well-defined event represented in the signal; determining the receiver ghost notch frequency from the amplitude/frequency spectrum of the signal in said window; and deriving from said receiver ghost notch frequency an estimate of the height of the water column above the sensor which produced the signal. The method may further include the steps of: identifying changes in arrival times from seismic signals received by a plurality of submerged acoustic sensors located at different offsets from an acoustic source; determining time differences between the identified changes in arrival times and expected changes in arrival times associated with an assumed acoustic sensor depth profile; and converting the time differences into depth differences between the assumed acoustic sensor depth profile and the actual depth profile of said acoustic sensors. This method provides for improved determination of local wave heights and acoustic sensor elevations and allows xe2x80x9cnoisexe2x80x9d in seismic data associated with changes in local wave heights and seismic sensor elevations to be attenuated during subsequent data processing.